We determined K-Ar ages and whole-rock chemistry for volcanic rocks from Azumaya volcano and the surrounding area of central Japan, in order to reexamine its volcanic history. Based on our chronological and petrological study, coupled with previous geological data, we reconstructed the volcanic history of the volcano. The volcano consists of three volcanic edifices (the Initial volcanic edifice, the Nekodake volcanic edifice, and the Urakurayama volcanic edifice). Volcanic activity started at ~0.8 Ma, forming the Initial volcanic edifice, which consists mainly of dacitic and basaltic andesitic lavas. At ~0.7 Ma, dacitic volcanic activity started around Nekodake and formed the Nekodake volcanic edifice within several tens of thousands of years. Activity of the Initial volcanic edifice ceased at ~0.55 Ma. After a dormant period of approximately 50,000 years, andesitic lavas effused from the eastern slope of the Initial volcanic edifice at ~0.5 Ma, forming the Urakurayama volcanic edifice and ceasing at ~0.45 Ma. Subsequently, a small volcano (Naruiwa volcano) erupted in the northwestern region of the Azumaya volcano at ~0.3 Ma.
Yamamoto et al. (2011) showed the occurrences of serpentinite in the Yaseo area on the Satsuma Peninsula, Kagoshima Prefecture, and interpreted them as klippes that were structurally emplaced onto the Cretaceous sediments of the Shimanto Belt. However, our investigation shows that the serpentinite was intruded sub-vertically into the Cretaceous sediments as sheets trending north–south, as proposed previously. This interpretation is supported by the observation that the serpentinite body at the top of a ridge, which Yamamoto et al. (2011) interpreted as a klippe, has sub-vertical boundaries with the Cretaceous sediments. In addition, the so-called debris and isolated blocks of serpentinite on the valley floor and mountainside, as shown by Yamamoto et al. (2011), are actually in situ outcrops of serpentinite. These in situ outcrops are the sites of waterfalls that fall to the valley floor and the sites of cliffs in the mountainside. The serpentinite bodies in the Yaseo area are correlated with those of Tokunoshima Island and western Shikoku, and were intruded from the latest Cretaceous to the earliest Paleocene.